Add --fields to notification & milestone listings (#422)

Together with #415 this finally adds the field flag to all entity listings.
closes #342

### ⚠️ breaking changes ⚠️
This changes the column names of `tea milestones ls`:

```diff
 - TITLE  | OPEN/CLOSED ISSUES | DUEDATE
 + TITLE  | ITEMS | DUEDATE
```

Co-authored-by: Norwin <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/422
Reviewed-by: delvh <dev.lh@web.de>
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

cmd/milestones/list.go

@@ -13,6 +13,10 @@ import (
 	"github.com/urfave/cli/v2"
 )
 
+var fieldsFlag = flags.FieldsFlag(print.MilestoneFields, []string{
+	"title", "items", "duedate",
+})
+
 // CmdMilestonesList represents a sub command of milestones to list milestones
 var CmdMilestonesList = cli.Command{
 	Name:        "list",
@@ -22,6 +26,7 @@ var CmdMilestonesList = cli.Command{
 	ArgsUsage:   " ", // command does not accept arguments
 	Action:      RunMilestonesList,
 	Flags: append([]cli.Flag{
+		fieldsFlag,
 		&cli.StringFlag{
 			Name:        "state",
 			Usage:       "Filter by milestone state (all|open|closed)",
@@ -37,10 +42,18 @@ func RunMilestonesList(cmd *cli.Context) error {
 	ctx := context.InitCommand(cmd)
 	ctx.Ensure(context.CtxRequirement{RemoteRepo: true})
 
+	fields, err := fieldsFlag.GetValues(cmd)
+	if err != nil {
+		return err
+	}
+
 	state := gitea.StateOpen
 	switch ctx.String("state") {
 	case "all":
 		state = gitea.StateAll
+		if !cmd.IsSet("fields") { // add to default fields
+			fields = append(fields, "state")
+		}
 	case "closed":
 		state = gitea.StateClosed
 	}
@@ -55,6 +68,6 @@ func RunMilestonesList(cmd *cli.Context) error {
 		return err
 	}
 
-	print.MilestonesList(milestones, ctx.Output, state)
+	print.MilestonesList(milestones, ctx.Output, fields)
 	return nil
 }